1. Field of the Invention
The present invention relates to a liquid tank and a fuel cell system and, more specifically, to a liquid tank (fuel tank) and a fuel cell system provided with such a tank, in which it is easy to monitor the remaining quantity of liquid fuel.
2. Description of the Related Art
A fuel cell is a device that generates electricity from hydrogen and oxygen and achieves highly efficient power generation. Unlike conventional power generation, a fuel cell allows direct power generation that does not require conversion into thermal energy or kinetic energy. As such, even a small-scale fuel cell achieves highly efficient power generation. Other features unique to a fuel cell include less emission of nitrogen compounds, etc. and environmental benefits due to small noise and vibration. As described, a fuel cell is capable of efficiently utilizing chemical energy in fuel and as such environmentally friendly. Fuel cells are envisaged as an energy supply system for the twenty-first century and have gained attention as a promising power generation system that can be used in a variety of applications including space applications, automobiles, mobile appliances and large and small scale power generation. Serious technical efforts are being made to develop practical fuel cells.
Of various types of fuel cells, a solid polymer fuel cell is unique in its low operating temperature and high output density. Recently, direct methanol fuel cells (DMFC) are especially highlighted. In a DMFC, methanol water solution as a fuel is not reformed and is directly supplied to an anode so that electricity is produced by an electrochemical reaction induced between the methanol water solution and oxygen. Reaction products resulting from the electrochemical reaction are carbon dioxide being emitted from an anode and generated water emitted from a cathode. Methanol water solution is richer in energy per unit area than hydrogen. Moreover, it is suitable for storage and poses little danger of explosion. Accordingly, it is expected that methanol water solution will be used in power supplies for automobiles, mobile appliances (cell phones, notebook personal computers, PDAs, MP3 players, digital cameras, electronic dictionaries (books)) and the like.
When a DMFC generates power, fuel is consumed. Thus, a DMFC system that uses a DMFC is supplied with fuel from a fuel tank which is detachable from the DMFC system and which is filled by pure methanol or highly-concentrated methanol water solution. When fuel in a fuel tank is exhausted, the fuel tank is replaced.    [Patent document No. 1]    JP 2004-127905 A
Conventional DMFC systems are designed to detect an empty fuel tank and suspension of the supply of fuel from the fuel tank to the DMFC system and notify a user that the fuel in the fuel tank is exhausted. Another conventional approach is for a DMFC system controller to calculate the total power generated since the fuel tank is attached so as to estimate fuel consumption from the total generated power. The user is alerted when it is desirable to replace a fuel tank.
Meanwhile, such a DMFC system requires complex measures for detecting an empty tank or detecting the remaining quantity.